Research On Characteristics Of Electromagnetic Ultrasonic Lamb Wave Testing

Electromagnetic ultrasonic Lamb wave detecting combines the characteristics of electromagnetic ultrasonic testing and guided waves testing,which has a better application prospect,due to its obvious advantages,such as no contact,detecting efficient and exciting convenient et.al.However,there are still some shortcomings in the electromagnetic ultrasonic Lamb wave testing.For example,it is difficult to analysis and to interpret the echo signals because of its multi-modes and dispersion,and the lower transducing efficiency of electromagnetic ultrasonic transducer makes the excitation signal is weak relatively,which reduces detection efficiency of Lamb waves in large component testing.Besides,the conventional linear ultrasonic testing technology is not sensitive to the microscopic defects in materials or structures.So,in order to solve the above problems,the characteristics of electromagnetic ultrasonic Lamb waves were analyzed in this paper.1)The magnetic circuits of existing permanent magnet were designed in order to improve the transducing efficiency of electromagnetic ultrasonic Lamb waves in the non-ferromagnetic materials.Two permanent magnet structures,which improve the static bias magnetic field in non-ferromagnetic materials,have been designed,and the transducing efficiency of EMAT has been increased,too.2)According to the Lamb wave mode shapes of S0 mode and A0 mode at low frequency and the excitation characteristics of the EMAT,the Lamb wave-mode control was realized through the selection of loading way of the EMAT.The choice of Loading way of the EMAT not only increase the excitation efficiency of the primary Lamb wave mode,but also can improve the suppression effect of the unwanted Lamb wave mode.3)In order to minimize the adverse influence of Lamb wave multi-mode on the subsequent signal analysis,double transducer excitation structure was proposed to excite single mode Lamb waves.4)The experimental platform was built and a series of analysis,such as the magnet structure design,Lamb wave mode control and double transducer excitation structure,were verified by experiments.To solve the problem that linear ultrasonic testing is not sensitive to the early damages,the nonlinear electromagnetic ultrasonic Lamb waves were proposed.In this paper,closed-crack defect was detected by electromagnetic ultrasonic Lamb waves on simulation analysis.The impacts of different parameters of crack on the higher harmonic were studied and the conditions of closed crack damage were analyzed through the change of the amplitude of higher harmonic.